This invention relates to a heat storage medium for use in a latent heat thermal energy storage unit.
The qualities which thermal energy storage units in general are required to posssess are large capacities for storage of heat, low price, long service life, and good heat transfer characteristics. Broadly thermal energy storage units (hereinafter referred to as "TES units") are divided into sensible heat TES units which make use of advantageous heat capacities and latent heat TES units which make use of latent heat as in fusion and solidification of substances or transition of crystals. In the former heat TES units, the capacities are determined by the temperature and heat capacity of the heat storage media used therein. To increase capacities, therefore, the TES units are required to either heighten the temperature of heat storage media or increase the volume of heat storage media to be used. In the latter heat TES units which make use of the heat of fusion of crystalline substances, since the heat storage media on fusion assume a liquid state, they require perfect separation between heat transfer media and heat storage media used therein and inevitably complicate their structures. If a heat storage medium to be used in a latent heat TES unit is processed so that it retains its original form even when it is softened during the fusion, then this heat storage medium can be kept in direct contact with the heat transfer medium. The use of this heat storage medium, therefore, siimplifies the TES unit's structure and enhances the heat transfer characteristics.
Crystalline polyolefins are inexpensive, possess high thermal stability, produce no corrosive actions upon containers and other vessels, exhibit no toxicity, and generate relatively large latent heat. These are excellent qualities for a heat storage medium in a latent heat TES unit. Nevertheless, they have a disadvantage that, even in a fused state, they are so viscous as to defy flowing motion by convection or agitation unlike ordinary liquids and further that, because of their low thermal conductivity, they cannot be effectively used in the form of large blocks. To overcome these disadvantages, there have been proposed a method which uses such crystalline polyolefins as molded in the form of pellets, rods or films and crosslinked so that they will retain their original forms even when they are exposed to contact with a heat transfer medium, and a method which uses the polyolefins as enclosed with capsules.
As for the aforementioned methods, there are methods disclosed in U.S. Pat. No. 4,182,398 which are indicated below:
(i) Crosslinking effected with a crosslinking agent such as a peroxide incorporated in polyolefins. PA1 (ii) Crosslinking effected by irradiation with electron beams. PA1 (iii) Using graft-polymerization to cover the surface of polyolefins with a silane which acts as a capsule.
The polyolefins, when crosslinked with a crosslinking agent incorporated therein, suffer degradation of the particular qualities which make them suitable for storage of latent heat. When the crosslinking of polyolefins is effected by irradiating polyolefins with electron beams, the polyolefins are crosslinked to the interior and are apt to suffer degradation of the qualities which make them suitable for storage of latent heat. The method which effects form-stability by grafting the surface of polyolefins with a silane is not economical because it requires large equipment and complicates the production process.